Openbox 3 themes are written as an X resource database, in a file named themerc. The format is quite simple to learn, but there are an enormous number of options available to you.
Each and every option has been documented, with details such as their default values, and which values are or are not valid for them. But to try make it a little more accessible, we'll start with an example theme, which uses every possible option within it. Each of the options can be clicked to read the details about it.